Generally, a disc-shaped optical recording medium (optical disc), such as CD (Compact Disc) and DVD (Digital Versatile Disc), is manufactured by attaching a stamper formed in a mastering process to a mold disposed within an injection molding machine, forming a disc-shaped substrate made of resin material by injection molding over an area forming an information recording surface, forming a recording layer capable of recording and a reflection layer capable of playing-back on the disc-shaped substrate, and forming thereon a protective layer made of resin.
The above discs are irradiated with a predetermined laser beam passing through the aforementioned disc-shaped substrate material, thereby performing information recording and/or playback.
Further, as disclosed in Japanese Patent Laid-Open Publication No. Hei 8-235638, an attention has recently been given to an optical disc which is manufactured by: first forming, as a disc-shaped substrate material (substrate), a thick support layer (protective layer) not required to be light-transmissible, i.e., not required to have an optical thickness by means of injection molding; and forming a reflection layer on the information recording surface side of the substrate for effecting a playback, or a recording layer or the like on that side for effecting a recording, followed by laminating a light transmitting layer based on a transparent resin layer which allows the passing of a laser beam for recording or playing back.
Conventionally, an optical disc has been manufactured in the following way. Namely, at the time of performing an injection molding to form a substrate, the center part of the substrate as well as a sprue runner solidified within the gate during the injection molding are removed so that a center hole is formed on the center part of the substrate so as to form a disc-shaped substrate material. Further, after forming the above reflection layer and the recording layer which has been constituted to be able to perform recording, a spin coating process is performed to form a protective layer made of a resin, or a light transmitting layer comprising a light transmitting resin.
As a characterizing method for manufacturing the aforementioned protective layer and the aforementioned light transmitting layer, Japanese Patent Laid-Open Publication No. Hei 10-249264 discloses that a lid member for covering the center hole of an optical disc is disposed, and then a resin is applied to the entire optical disc over the lid member while the disc rotates and solidified thereafter, thus forming a protective layer (resin layer). Since this method can easily control the thickness of the resin layer within an applying surface such that a substantially uniform state is achieved in the radial direction, the same method can also be used in a light transmitting layer formation process disclosed in Japanese Patent Laid-Open Publication No. 1996-235638.
In order to use the above-described resin layer rotational applying method to form a resin layer in a more uniform state, it is necessary to have a lid member to block the center hole of a disc-shaped substrate material.
When using such a manufacturing method, it is desired that the lid member be clean. When a single lid member is used, the lid member must be cleaned after every use in resin coating. Alternatively, the lid member must be disposable, so that a new (clean) lid member is always used.
Further, as described above, when the center hole is closed by the lid member during the spin coating process, since the light transmitting resin usually flows downward from the lid member to the disc-shaped substrate, air bubbles are apt to occur within the resin layer and irregular flow traces are likely to appear on the surface of the resin layer, all due to a difference in height between the lid member and the surface of the disc-shaped substrate, hence resulting in non-uniform thickness of the resin layer (non-uniform film thickness).
On the other hand, as disclosed in Japanese Patent Laid-Open Publication No. Hei 5-92492 as well as in Japanese Patent Laid-Open Publication No. Hei 5-185477, there have been suggested optical disc manufacturing methods in which the center hole of a disc-shaped substrate is formed immediately after the formation of the disc-shaped substrate itself, instead of forming such a center hole during the formation of the disc-shaped substrate, thus minimizing an influence from burr or resin waste generated due to the punching of the center hole.
Since the aforementioned methods aim at minimizing an influence from burr or resin waste, they are characterized in that the center hole is formed immediately after the formation of the disc-shaped substrate, with the sprue runner and the center part being removed simultaneously prior to the formation of a light transmitting resin layer.
In view of the above, the applicant of the subject application has already suggested an optical disc manufacturing method (not publicly known) which is simpler and has a higher yield in forming resin layer than a conventional manufacturing method using a lid member.
According to the suggested method, during the injection molding which is one step in manufacturing an optical disc, a disc-shaped substrate material is taken out integrally with the center part of a disc-shaped substrate and the sprue runner, passed through a film formation step and a resin layer formation step, followed by simultaneously removing the center part and the sprue runner as a last step, thereby forming the optical disc.
According to the above-described method for manufacturing a disc-shaped optical recording medium in which the center part is punched at the last step to form the center hole which is a chucking hole, during the step of punching the center hole, burr will generate along the hole edge in the thickness direction of the formed center hole. In this case, once the disc-shaped optical recording medium is loaded onto a drive, there will be a problem that an eccentricity can occur during the rotation of the recording medium due to such burr.
Moreover, when the center hole is punched at the last step of the above-described manufacturing process, if the disc-shaped substrate material is set eccentric with respect to a tool such as an ultrasonic press, the center hole will deviate from the center of the disc-shaped optical recording medium, resulting in a problem that it is difficult to perform a correct information recording or playback.